Subject: [Isis-wg] Comments on draft-ietf-isis-wg-multi-topology

From: stefano previdi <[email-masked]>
Date: Mon, 22 Jul 2002 19:17:29 +0200

Hi Naiming,

just a few comments on draft-ietf-isis-wg-multi-topology. 

> 9.  MT SPF Computation
> 
>    Each MT MUST run its own instance of the decision process.
>    The LSP overload bit and pseudo-node LSPs are used by all topologies
>    during computation.  Reverse connectivity check within SPF MUST
>    follow the according MT to assure the bi-directional reachability.
>    The results of each computation MUST be stored in a separate RIB,
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>    otherwise overlapping addresses in different topologies could
>    lead to undesirable routing behavior such as forwarding loops. 

I believe that use of overlapping addresses will require more than just 
separate RIBs. When the router will receive an IP packet, it will have to use 
some other mechanisms (than just IP destination address lookup) in order to 
determine which RIB it has to use to forward such packet (especially when you 
play with multiple topologies in the same address family).

Therefore I found the following statement: "MUST be stored in a separate RIB" 
a bit extreme and not totally correct. It's not needed in all cases and, in 
the cases where it's needed (assuming overlapping addresses are used), it's 
not enough to ensure correct forwarding.

Of course if you use a separate set of adjacencies/interfaces for each 
topology, then you're safe... but this means a complete overlay network and I 
believe it's not the scope of the MT-ISIS draft...

So, I would suggest to drop the "MUST be stored in a separate RIB" and add a 
more generic statement saying that if multiple RIBs are used (because of 
overlapping addresses), then the forwarding layer MUST have a mechanism 
allowing correct RIB selection for incoming packets.

> 9.1 Generic MT SPF (GSPF) for Network Management

[...]

>    Implementations SHOULD allow the administrator to control the
>    setting of MT Management Color to specific prefixes of the router
>    and the setting of Generic MT SPF to install the MT Mangement
>    colored routes into specific RIB. It is recommended for the
>    administrator's to ensure consistent configuration of all routers
>    in the domain to prevent undesirable forwarding behavior.

####
Same here. If you use unique addresses for your management MT, then you don't 
need a separate RIB. Otherwise you also need the forwarding layer to decide 
which RIB to use.... and in this particular case it's not trivial.
####

s.

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